We have generated monoclonal antibodies against human breast cancer tissue culture cells. One of our antibodies, H59, generated against ZR-75-1 cells, may react with a cell surface protein present in only breast cancers which contain estrogen receptor. The goal of this project is to determine if the monoclonal antibody detects an antigen which is estrogen regulated. The proof will be derived from four independent approaches: (1) measurement of the antigen in human hormone dependent breast cancer cell lines during conditions of estrogen stimulation and anti-estrogen inhibition; (2) measurement of the effect of antigen modulation on estrogen mediated events in breast cancer cells; (3) correlation of H59 antibody binding with estrogen and progesterone receptor values in human tumors; and (4) correlation of H59 binding with a patient's clinical response to hormone therapy. The antigens which bind H59 and a second monoclonal antibody which reacts with a hormone independent antigen (H71) will be measured by radioimmunoassay in two human estrogen responsive breast cancer cell lines, ZR-75-1 and MCF-7, under conditions of estrogen stimulation and tamoxifen inhibition. Since H71 antigen is hormone independent, it should not be affected except by conditions which affect cell growth. H71 will serve to discriminate specific induction of H59 antigen from somatotrophic effects of hormones. Antigen modulation will be determined by comparing the effects of H59 and H71 binding to breast cancer cells on the known hormone regulated events of macromolecular synthesis and progesterone receptor content. The antibody binding to tissue specimens will be correlated with the presence and levels of estrogen receptor as determined with the standard dextran coated charcoal assay and as determined by nuclear binding assays, exchange assays, and estrogen receptor monoclonal antibody binding to fixed tissue specimens. Patients whose tumors contain H59 antigen will be followed longitudinally to determine if the presence of the antigen predicts response to sex steroid hormone therapy and eventually the presence of metastatic disease. These studies will determine if H59 antigen is hormone dependent.